In a known structure of a membrane electrode assembly (MEA) used for a polymer electrolyte fuel cell, a porous layer is stacked between a catalyst layer and a gas diffusion layer (see, for example, Patent Literatures 1 and 2). The porous layer used in this structure is a layer comprised of continuous micropores and is also called a microporous layer (MPL).
Patent Literature 1 describes producing a porous layer by applying a paste containing conductive carbon powder, metal fiber and a non-polymer fluorine material on a catalyst layer or a gas diffusion layer.
Patent Literature 2 describes producing a porous layer from a mixture of conductive carbon powder and polytetrafluoroethylene (PTFE) powder. Patent Literature 2 also describes producing a porous layer from coprecipitate that is obtained from a mixed solution containing conductive carbon powder and PTFE powder.